This invention relates to a protective coating composition. More particularly, it relates to a silicone resin coating composition with improved storage capacity which, when applied to a substrate, forms a protective, abrasion-resistant coating thereon.
Recently, the substitution of glass glazing with transparent materials which do not shatter, or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles such as trains, buses, taxis and airplanes. Lenses in eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant, transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a factor in its fuel economy.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature and good dimensional stability. It is also self-extinguishing and is easily fabricated. Acrylics, such as polymethylmethacrylate, are also widely used transparent plastics for glazing.
Although these plastics provide the advantages of being more shatter-resistant and lighter than glass, they are easily marred and scratched by contact with abrasives such as dust, cleaning equipment and ordinary weathering. This has led to the development of coatings which serve to improve the abrasion resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225 (Misch et al.), 3,986,997 (Clark), 3,976,497 (Clark) and 4,177,315 (Ubersax), for example, describe such compositions.
In copending U.S. application Ser. No. 964,910, now abandoned, coating compositions having improved resistance to moisture and humidity and ultraviolet light are disclosed. It was discovered therein that, in direct contrast to the teachings of U.S. Pat. No. 3,986,997, compositions having a basic pH, i.e., 7.1-7.8, do not immediately gel but in fact provide excellent abrasion-resistant coatings on solid substrates.
Silicone resin coatings such as those disclosed in U.S. Pat. No. 4,309,319, incorporated herein by reference, have shown great promise in protecting transparent plastics. However, a serious drawback diminishing their usefulness is that silicone resin hard coatings are short-lived when stored at room temperature. Refrigerated storage and shipping are required to obtain extended shelf life beyond the usual one-month life at room temperature.